Generally, a radio receiver uses an IF filter, which applies band limitation to an intermediate frequency signal (IF signal) generated by performing frequency conversion of a received high-frequency signal (RF signal) in order to generate an intermediate frequency signal including a desired reception frequency. A band-pass filter is used for the IF filter.
When creating a band-pass filter having a large Q, a biquad type filter is often used. The biquad type filter is a 2-dimensional filter constructed of three operational amplifiers, which is highly accurate in its center frequency and allows for a considerably large Q value.
In addition, the band-pass filter can be constructed as a combination of a high-pass notch filter (HPN) and a low-pass notch filter (LFN) (e.g., refer to patent document 1: Japanese Patent Laid-Open No, 2001-77716).
That is, as illustrated in FIG. 1, it is possible to constitute a band-pass filter with a center frequency fo by combining a high-pass notch filter having a high-pass notch point frequency fH with a low-pass notch filter having a low-pass notch point frequency fL. It is possible to change a band width of a pass band by adjusting the high-pass notch point frequency fH and the low-pass notch point frequency fL.
A notch filter can be constructed of a low-pass filter, a high-pass filter, and an adder for adding an output of the low-pass filter and an output of the high-pass filter. Here, a BEF (Band Elimination Filter) with a high-pass notch can be constructed by combining the high-pass filter having a high Q with the low-pass filter having a low Q, and a BEF with a low-pass, notch can be constructed by combining the high-pass filter having a low Q with the low-pass filter having a high Q.
When a band-pass filter is constructed using a time-series analog filter by CMOS process, an RC active filter, a Gm-C filter, a MOSFET-C filter, or the like is used for the analog filter. The RC active filter is constructed using an operational amplifier, a resistor and a capacitor, which applies to an IF filter amplifier described in the above-mentioned patent document 1. The Gm-C filter is a filter constructed of a Gm amplifier, which converts an input voltage signal into an output current signal, and a capacitor, and can change characteristics of the filter by changing transconductance Gm. The MOSFET-C filter is a filter constructed by combining a MOSFET, which is used as a resistor, with a capacitor.
FIG. 2 is a diagram illustrating an example of a conventional filter circuit constructed using a notch filter with a Gm-C filter. A filter circuit illustrated in FIG. 2 is constructed having an OTA (Operational Transconductance Amplifier) 101, a high-pass notch filter (HPN) 102, and a low-pass notch filter (LPN) 103. The HPN 102 is constructed by combining a high-pass filter having a high Q with a low-pass filter having a low Q. In addition, the LFN 103 is constructed by combining a high-pass filter having a low Q with a low-pass filter having a high Q.
An adder provided on the LPN 103 operates as an Output buffer amplifier. On the other hand, the OTA 101 is used as a buffer amplifier on an input side of the HPN 102. The OTA 101 is a current output type because of its circuit structure and has a large output impedance. On the other hand, the HPN 102 connected with an output of the OTA 101 is equipped with a high-pass filter having a high Q as mentioned above. When this is constructed of a Gm-C filter, a capacitance of a capacitor connected, with an input stage becomes small.
Nevertheless, when a high-pass filter having a high Q is connected with the output of the OTA 101 having a large output impedance, a buffer effect of the OTA 101 becomes not so expectable under the influence of the capacitor having a small capacitance and being connected with the input stage of the high-pass filter. This may result in multifeedback, and a problem of a zero point of a BEF deviating from a design value and a problem of oscillation and the like are likely to arise. These problems arise also when a current output differential amplifier is used instead of the OTA 101.